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Myrmekite and Strain Weakening in Granitoid Mylonites

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Myrmekite and Strain Weakening in Granitoid Mylonites Myrmekite and strain weakening in granitoid mylonites Alberto Ceccato1*, Luca Menegon2, Giorgio Pennacchioni1, Luiz Fernando Grafulha Morales3 1 Department of Geosciences, University of Padova, 35131 Padova, Italy 2 School of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UK 5 3 Scientific Centre for Optical and Electron Microscopy (ScopeM) - ETH Zürich, Switzerland Correspondence to: Alberto Ceccato ([email protected]) * Now at: School of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UK Abstract. At mid-crustal conditions, deformation of feldspar is mainly accommodated by a combination of fracturing, dissolution/precipitation and reaction-weakening mechanisms. In 10 particular, K-feldspar is reaction-weakened by formation of strain-induced myrmekite - a fine-grained symplectite of plagioclase and quartz. Here we use EBSD (i) to investigate the microstructure of a granodiorite mylonite, developed at ~ 450 °C during cooling of the Rieserferner pluton (Eastern Alps), and (ii) to assess the microstructural processes and the weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of plagioclase in pristine 15 myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain size reduction and anticlustered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws, calculated for monomineralic quartz, feldspar, and 20 quartz + plagioclase aggregates (sheared myrmekite) during deformation at 450 °C, show that grain- size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the felsic middle crust. 25 1. Introduction Localization of ductile strain within rocks arises from weakening associated with grain size refinement processes by dynamic recrystallization, metamorphic reactions, and microfracturing (e.g. Platt et al., 2015, and reference therein). Grain size reduction, accompanied by phase mixing in polymineralic rocks at high strains, commonly results in a switch of deformation mechanism from grain-size- 30 insensitive (GSI) to grain-size-sensitive (GSS) creep – one of the most effective strain weakening mechanisms within shear zones (Kruse and Stünitz, 1999; Kilian et al., 2011; Menegon et al., 2013). 1 Feldspars locally form the load-bearing framework of crustal rocks (Handy, 1994). At mid-crustal conditions, feldspar deformation mainly occurs by microfracturing and dissolution/precipitation processes, typically associated with metamorphic reactions (Behrmann and Mainprice, 1987; Michibayashi, 1996; Stünitz and Tullis, 2001; Gueydan et al., 2003; Ree al., 2005). Kfs (K-feldspar) 5 is commonly replaced by myrmekite – a fine-grained symplectic aggregate of Qtz (quartz) and Plg (plagioclase) (Becke, 1908; Vernon, 1991). Myrmekite replacement is either related to Kfs chemical instability (Cesare et al., 2002), in some cases involving local metasomatic fluids (Phillips, 1980), or triggered by stress concentration and intracrystalline strain in Kfs during deformation (Simpson and Wintsch, 1989; Menegon et al., 2006). This replacement is acknowledged as a weakening mechanism 10 during ductile deformation of granitoid rocks (LaTour and Barnett, 1987; Simpson and Wintsch, 1989; MacCaffrey, 1994; O’Hara et al., 1997; Tsurumi et al., 2003; Pennacchioni, 2005; Menegon et al., 2006; Pennacchioni and Zucchi, 2013; De Toni et al., 2016). Deformation and shearing of myrmekite result in a fine-grained Plg + Qtz aggregate, that is manifestly weaker than original coarse Kfs (Tsurumi et al. 2003; Ree et al., 2005; Ciancaleoni and Marquer, 2006). In general, a fine grain size 15 and the local presence of grain boundary aqueous fluid promote phase mixing and the development of ultramylonites (Vernon, 1991; Kilian et al., 2011; Czaplińska et al., 2015). Though the key role of myrmekite in strain localization has been recognized, it has not been accompanied with a quantitative analysis of the deformation mechanisms within myrmekite-derived, fine-grained Plg + Qtz aggregates. Here we present a detailed analysis of myrmekite evolution, from the nucleation stage within Kfs to 20 the development of sheared Plg + Qtz aggregates, and of the associated rheological weakening that resulted in strain localization in the mylonites of the Rieserferner granitoid pluton (Eastern Alps). In this pluton, ductile shear zones nucleated along joints that were locally filled with Qtz and Ep (epidote) veins during post-magmatic cooling (Ceccato et al., 2017; Ceccato and Pennacchioni, 2018). The progressive development of granodiorite mylonite was associated with consumption of Kfs by 25 myrmekite leading to increasingly interconnected, fine-grained Plg + Qtz layers. Microstructures of granodiorite mylonite have been analysed to characterize: (i) the process of myrmekite nucleation; (ii) the deformation mechanisms during myrmekite shearing and transition to Plg + Qtz aggregates; (iii) the deformation mechanisms of pure Qtz layers; (v) the deformation mechanisms of Kfs porphyroclasts and of Kfs new grains during mylonitization. Furthermore, the application of mixed flow laws of the 30 aforementioned deformation mechanisms for polymineralic aggregates allows the degree of rheological weakening resulting from deformation of myrmekite to be quantified. The current paper represents a first attempt at quantitatively estimating, based on a robust microstructural analysis, the rheological effects of the development of myrmekite during mylonitization of granitoids, and at 2 determining the deformation mechanisms in syn-kinematic monomineralic and polymineralic aggregates at mid-crustal conditions. This estimate was not attempted yet. This analysis is validated by comparison with experimental data on deformation of poly-phase mixtures. 2. Geological setting and field description 5 The tonalitic-granodioritic Rieserferner pluton (Eastern Alps) (Bellieni, 1978) was emplaced at ~15 km depth (0.4 GPa; Cesare et al., 2010) into the Austroalpine nappe system at 32 Ma (Romer and Siegesmund, 2003). During post-magmatic cooling, a main set of ductile shear zones exploited shallowly ESE-dipping joints, and the joint-filling Qtz and Ep veins (set 2 of Ceccato and Pennacchioni, 2018). The temperature of ductile shearing has been estimated at 420-460 °C based on 10 thermodynamic modelling (Ceccato, 2018). Ductile shearing along joints and Ep-filled joints resulted in cm-thick heterogeneous shear zones with a sigmoidal-shaped foliation in the host granodiorite (Ceccato and Pennacchioni, 2018) likely reflecting fluid-rock interaction at the vein selvages (Pennacchioni and Mancktelow, 2018). In contrast, Qtz veins filling the joints sharply localized homogeneous shearing (Ceccato et al., 2017). 15 In this study we analyse a sample of mylonitic shear zone within the Rieserferner granodiorite (sample ID: 10-019A; sample coordinates: N 46°55'24.8" E 12°07'36.2"). The structural, mesoscale context of the studied sample is well illustrated by Ceccato and Pennacchioni (2018) and in particular by the map in Fig. 4 of their supplementary online material. 3. Analytical methods 20 Polished thin sections of granodiorite mylonite were prepared for the study of the microstructure and of the crystallographic preferred orientations (CPO). The thin sections were made from rock chips cut parallel to the stretching lineation and perpendicular to the shear plane (XZ plane of finite strain ellipsoid). Electron backscattered diffraction analysis was carried out on a JEOL 7001 FEG SEM equipped with 25 a NordLys Max EBSD detector (AZTec acquisition software, Oxford Instruments) at the Electron Microscopy Centre of Plymouth University. EBSD patterns were acquired on rectangular grids with step sizes of 0.2, 0.3 and 0.35 m. Working conditions during acquisition of EBSD patterns were 20 kV, 70° sample tilt, high vacuum, and working distance between 17 and 23 mm. A detailed description of the EBSD post-processing methods and of the image analysis are reported in Appendix A. The 3 microstructural and CPO analysis conducted with EBSD were complemented with cathodoluminescence (CL) and microchemical analyses. CL imaging was performed in a FEI Quanta 200 FEI equipped with Gatan monocle detector. Imaging was performed using an accelerating voltage of 20 kV, beam current of 8 nA and working distance of 5 20 mm in C-coated (15 nm) thin sections used for EBSD analysis. To avoid incorrect interpretation of potential artefacts in the sample, secondary (SE) and backscatter electron images were collected simultaneously with CL. Microchemical analyses were performed with wavelength-dispersive spectroscopy (WDS) at Electron Microprobe Laboratory at the Università degli Studi di Milano with a Jeol 8200 Super Probe; the 10 operating conditions were: 15 kV accelerating voltage; 5 nA (Kfs and Plg) beam current. PAP correction program was applied to convert X-ray counts into oxide weight percentages. 4. Microstructure
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